Gab Kovacs1 and Paula Briggs2
(1)
Department of Obstetrics and Gynaecology, Monash University, Clayton, Victoria, Australia
(2)
Sexual and Reproductive Health, Southport and Ormskirk Hospital, Southport, UK
Chromosome Structure
Genetic Abnormalities
Single Gene Defects
Aneuploidy
Multifactorial Congenital Abnormalities
Antenatal Screening for Chromosmal Problems (Prenatal Diagnosis –PND)
Preimplantation Genetic Diagnosis (Fig. )
The basic makeup for every individual is determined by their genes. Genes are made up of a sequence of nucleotides, which are aggregated into chromosomes. The Human Genome is believed to be made up of about 23,000 genes, located in 23 pairs of chromosomes. Each species is unique in respect of the number of chromosomes that it is comprised of. The chromosomes are contained within the nucleus of every cell. In any individual the chromosomes are identical in each of its 37.2 trillion cells. As the 23,000 pairs of genes can form an infinite number of combinations and permutations (23,000 × 23,000), each individual is genetically unique with the exception of identical twins, who are formed by the splitting of an embryo, and thus start with identical genetic makeup.
Chromosome Structure
Chromosomes are a joined by a centromere. As the centromere is closer to one end, the shorter arms are called p arms and the longer arms q, and the end region is the telomere (Fig. 3.1).
Fig. 3.1
Chromosome structure
Within the nucleus, the chromosomes are in pairs, one of each pair from each parent, containing the genes present, one inherited from the male partner via the sperm and one from the female partner via the oocyte. These genes determine our characteristics. All cells of the body (somatic cells) contain these 46 (23 pairs) of chromosomes (diploid) except the germ cells (oocyte and spermatid) which contain only 23 chromosomes (haploid).
All oocytes carry 22 autosomes, with the 23rd chromosome always being an X. In contrast, sperm carry either an X or a Y bearing 23rd sex chromosome, and the 22 autosomes. Sperm that carry the Y chromosome are destined to produce a male offspring when it combines with an ovum, whereas and X bearing sperm will produce female offspring. The Y chromosome is significantly shorter than the X chromosome (Fig. 3.2).
Fig. 3.2
Karyotype
The crucial difference between germ and somatic cells is that the former reduces the diploid (46 chromosome) compliment to haploid (23 chromosome) form, thus enabling the joining of two haploid germ cells to form one new diploid individual.
During the proliferation of germ cells, we have two types of cell division, mitosis (or replication division) and meiosis (reduction division).
In the male germ cell, this reduction division occurs during its progression from the primary to secondary spermatocyte, whilst in the female from the primary oocyte to the secondary oocyte.
The female gamete is unique in that division of the primary oocyte results in uneven division of the cytoplasm, with most going to the daughter cell, the secondary ooctye, with only a very small amount going to the other daughter cell, which becomes the primary polar body. Similarly, when the secondary oocyte divides by mitosis, replicating its 23 chromosomes, one of the daughter cells again is disadvantaged with respect to the amount of cytoplasm, and it becomes the second polar body, whereas the other part becomes the mature oocyte. The polar bodies are not capable of being fertilized.
The sex of an embryo is determined at the time of fertilization and depends on whether an X bearing or Y bearing sperm has entered the ovum.
Genetic Abnormalities
Genetic abnormalities may result from loss or extra chromosomal material or an abnormality of a single gene. Chromosome abnormality may involve part of or an entire chromosome. For example Down Syndrome, where the child inherits an extra chromosome 21, known as trisomy 21. Genes can be altered in a variety of ways. The outcome depends on where the change is in the gene, how it effects gene function and how the gene change is inherited.
Single Gene Defects
Single gene defects can be dominant or recessive. With a recessive disorder, both genes (one from each parent) have to be altered in order for the abnormality to be manifest (phenotype). If only one parent has passed on the defective gene, and the same gene from the other parent is normal, the child will not be affected, but will be a carrier. In contrast, with a dominant single gene defect it is sufficient for just one gene change to be inherited, and that gene will be responsible for the phenotype, e.g. Huntington’s disease.
If the gene is on the X chromosome, the outcome is gender specific. For example the Haemophilia Gene. In females, if the X chromosome carries the defective gene, there is a second X chromosome which caries a normal gene and the carrier has normal clotting. A male however, only has one X chromosome nd if his Haemophilia gene is altered, he has Haemophilia.
(If a person carries two altered genes he/she is said to be homozygous, whereas if they carry one normal and one abnormal gene, they are said to be heterozygous.)
Autosomal Dominantly Inhherited Single Gene Defects
· One altered gene is sufficient to cause the phenotype
· An affected offspring may have an affected parent unless the gene change is a new mutation
· Male and female offspring have the same chance of inheriting it
· If an affected person has a child with an unaffected person, the risk of affected children is 1:2
· Unaffected children of an affected parent will not carry or pass on that gene.
Autosomal Recessively Inherited Single Gene Defects
· Both parents are carriers and both copies of the gene are altered in the child and disease to be manifest
· Heterozygous carriers are phenotypically normal, but carry and may pass on, the altered gene
· If normal parents have an affected child, they must both be heterozygotes- 1:4 children will be affected, 1:2 carriers, 1:4 not carriers, -or conversely 3:4 will be unaffected.
· If both parents affected, all their children are affected
· If an affected parent (homozygous) has a child with a heterozygous carrier- 1:2 children will be affected, 3:4 carriers
· X linked recessive traits, will be manifest in all males, as they only have one X chromosome
· Heterozygous females are nearly always normal, but are carriers and pass on the gene to 1:2 of offspring
· Of a carrier female, there is a 1:2 chance that sons will be affected and 1:2 chance that the daughters will be carriers
· An affected male never passes the gene on to a male offspring, since it is the Y chromosome (not the X) that he passes to his son
· All daughters of an affected male are carriers
· The female child of a carrier female and a normal father has a 1:2 chance of being affected
Aneuploidy
These abnormalities arise when the number of chromosomes is less or more than 46.
Trisomies
The most common type of aneuploidy is Downs Syndrome, Trisomy 21, occurring in about 1:650. The aetiology is thought be an abnormal separation of chromosomes during the first and sometimes second meiotic division in the ovum. It is maternal age dependent with the incidence at various ages shown in Table 3.1
Table 3.1
Age at conception by incidence of Trisomy 21 (USA)
|
Age at conception |
Incidence |
|
20 |
1:1,600 |
|
25 |
1:1,300 |
|
30 |
1:1,000 |
|
35 |
1:365 |
|
40 |
1:90 |
|
45 |
1:30 |
Other common trisomies are of chromosome 13, 16, 18 and 22, but most of these are incompatible with life and result in early pregnancy loss.
A special trisomy is that of 47XXY or Klinefelter Syndrome, with an incidence of 1:450 of males. The phenotype is variable. They may first come to attention as adults because of infertility. Men with Klinefelter syndrome may be tall, with breast development, and they may have problems with learning and language and because of testosterone deficiency, have minimal facial hair, decreased libido, and lack of energy.
On examination they usually have small testes (as little as 3cc in volume), high serum FSH and low testosterone.
Monosomies
Monosomy, where there is a missing chromosome is incompatible with life, except for when it occurs in the sex chromosome, presenting as 45 X, or Turner Syndrome. It is estimated that 90 % of babies with Turner Syndrome are spontaneously aborted in early pregnancy. It effects about 1:2,000 girls at birth. These girls are usually short in stature (average height only 143 cm), have delayed puberty, and often primary amenorrhoea, or very infrequent periods. They often have webbed neck, and may have learning difficulties. They are almost always sterile. About 50 % have an associated congenital cardiac defect, and some have hearing problems. They may have puffy hands and feet, pigmented moles, soft spooned nails and a low hairline. Frequently they have XO/XX mosaicism, meaning that some cells have a 46XX karyotype, while others have the true 45 X.
Translocations
In this congenital abnormality, part of one chromosome becomes attached to or interchanged with another chromosome or segment thereof. If there is a rearrangement, but no loss or gain of genetic material, it is called a balanced translocation, and may have no phenotypic significance. However if genetic tissue is lost or gained, resulting in an unbalanced translocation, this often results in an early pregnancy loss (EPL). In the event that the pregnancy continues the degree of severity of any congenital anomality depends on the specific break points involved, and the amount of unbalanced genetic tissue.
Multifactorial Congenital Abnormalities
There are some abnormalities that are believed to have a combined inherited and environmental mode of transmission. Although no specific single gene has been identified, these abnormalities do run in families. An example is neural tube defects, where the neural tube does not close properly, resulting in anencephaly (at cephalic end) and spina bifida (at caudal end). These are the second commonest group of congenital abnormalities after cardiac defects. With the use of antenatal ultrasound examination, these abnormalities are usually diagnosed in the late first or mid trimester, thus allowing the option of therapeutic termination of pregnancy (TOP).
Antenatal Screening for Chromosmal Problems (Prenatal Diagnosis –PND)
During the last two decades antenatal diagnosis has seen many new options, and widespread availability in developed countries. These tests can be divided into screening tests and diagnostic tests.
Screening tests are directed at predicting the risk of aneuploidy, principally trisomy 21, but also other chromosomal aneuploidies. The most popular is a combined first trimester blood test measuring two hormones in the blood, (free-BhCG and PAPP-A), in conjunction with ultrasonic measurement of the fetal nuchal thickness. A computer program, then uses the results, together with maternal age to estimate the likely risk. If the risk is elevated, a diagnostic test is recommended.
Non Invasive Prenatal Screening (NIPS/ Non Invasive Prenatal Testing (NIPT) has recently been introduced, where blood taken from the mother is processed to isolate cell free foetal DNA that has passed through the placenta. This test can diagnose most trisomies and monosomy for chromosmes 18 (Edwards Syndrome) and X (Turner’s Syndrome). Although all tests do have false positive and false negative results, NIPS is estimated to identify 98 % of Downs Syndrome, with a false positive rate of less than 0.5 %.
Diagnostic Tests
If the screening test is suggestive of a significant risk of a chromosomal abnormality. A confirmatory diagnostic tests is recommended and most will return a normal result.
Chorionic Villus Sampling (CVS) (Fig. 3.3)
Fig 3.3
Chorionic villus sampling
This procedure is performed at 11–12 weeks of gestation under ultrasound guidance and enables a sample of the chorionic villi to be biopsied. Foetal cells are then isolated, with subsequent confirmation of euploidy (normal chromosome complement) or aneuploidy. The estimated risk of pregnancy loss after the procedure is about 1:100.
Amniocentesis (Fig. 3.4)
Fig 3.4
Amniocentesis
This technique is similar to CVS, except that it is performed after 13 weeks of pregnancy, and a sample of amniotic fluid (rather than placental tissue) is collected. Analysis is similar to CVS, and the risk of pregnancy loss post procedure is 1:200.
Preimplantation Genetic Diagnosis (Fig. 3.5)
Fig 3.5
Embryo biopsy for preimplantation genetic diagnosis
Waiting till a pregnancy is achieved, and then undertaking PND means that if the foetus is affected, then therapeutic TOP may have to be undertaken. Many couples find this unacceptable and prefer not to establish a pregnancy with an abnormal foetus. The alternative is to produce embryos in vitro, using conventional in vitro fertilization (IVF) treatment, biopsying the early embryo, and then carrying out genetic diagnosis, either looking for chromosomal abnormalities, or single gene defects, if there is a known gene defect in the family. Only embryos which are diagnosed to be unaffected are then transferred. Sometimes the embryos have to be frozen and replaced in a subsequent cycle.
This is a simplified review of genetic diagnosis, but genetic counselling is complicated and is best carried out by a multi-disciplinary team, including geneticists, genetic counsellors, obstetricians and midwives.